This invention relates to an apparatus for manufacturing ribbed pipes with a smooth inner surface from a mouldable material, said apparatus comprising a core including a shaft, a conically widening mandrel located downstream of the shaft seen from the direction of production of the apparatus, and a kernel of a substantially uniform diameter disposed after the mandrel, an extrusion sleeve surrounding the core and forming with the core a nozzle for the material, the extrusion sleeve having an end face located upstream of the kernel seen in the direction of production of the apparatus, and moulds encasing the extrusion sleeve and the core and being movable along an endless path, having grooves on the inner surface for forming ribs on the pipe, whereby an initial zone of the kernel which is in the vicinity of the nozzle has means for heating this zone and an end zone of the kernel which is remote from the nozzle has means for cooling this zone.
The production of closed-wall ribbed pipes having prominent and narrow ribs and a smooth inner surface has proved unexpectedly difficult. Prominent and narrow ribs would be the most advantageous in view of the use of the pipe. In order for the deep and narrow grooves in the inner surface of the moulds to be filled, high pressures must be used which place severe requirements on the endurance of the apparatus. These problems are aggravated when a stiff plastic, such as unsoftened PVC, is used.
Another important problem relates to the quality of the inner surface of the pipe. The smoothness of the inner surface is impaired e.g. by the following factors: air bubbles generated in the forming space, melt tears resulting from a high shearing rate, irregularities and cold seams resulting from the laminar structure of the material, the adhesion of the inner surface to the kernel, and depressions and air bubbles in the material resulting from the uneven cooling of the mass.
The publication WO 87/04113 discloses an apparatus wherewith it is possible to manufacture ribbed pipes of a relatively high quality. In this known apparatus the kernel is divided into two zones whereof an initial zone located closest to the nozzle is heated and an end zone located after it is cooled.
The reference discloses that the diameter of the initial zone of the kernel increases slightly in the direction of movement of the material. By heating the pipe in the region of the kernel and simultaneously forcing the material toward the moulds, the material is caused to fill the grooves in the moulds simultaneously as the inner surface of the pipe is made fully smooth. This effect can probably be explained by the fact that after the extrusion pressure has filled the grooves in the moulds in the region of the conical mandrel, the material begins to cool by the action of the moulds and thereby also to shrink. Imperfections due to different cooling rates and shrinkage of the material are prevented by generating a considerable afterpressure in the grooves of the moulds, on account of which the material to be formed closely follows the surfaces of the moulds and the kernel. This afterpressure is independent of the extrusion pressure and can therefore be easily regulated by varying the heating effect.
However, in practice it has been found that the inner surface of pipes manufactured by this known apparatus is not always smooth but contains irregularities.